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drilled hole tolerances. On turning centres this can be readily achieved by modifying the CNC cutting pro- gram, to oset the drill with respect to the machine’s centreline. Moreover, for turning operations employ- ing drilled features, then the indexable insert’s top sur- faces must remain parallel to the X-axis of the machine tool. e arrangement of the inner and outer cutting edges of an indexable insert drill relative to each other, together with the drill’s position to the axis of rotation are vital for perfect drilling operations 19 (i.e. see Fig. 53a). e cutting inserts positions by possible X-axis adjustments, are critical for the: smooth running, re- sultant cutting forces and, will inuence the drilled hole’s alignment. Preferably, the cutting edges are ar- ranged in such a manner that the inner- (SBI) and outer-inserts (SBA) have identical cutting widths (Fig. 53a – bottom le). When new insert cutting edges are utilised, this results in a balance of the cutting forces in the Y-axis, guaranteeing drilled holes of accurate size and surface texture without ‘retraction striae’ 20 . When selecting the appropriate adjustment angles (χ i ) and (χ A ), the lines of force via feed forces (F a and F i ) will co- incide with the drill’s axis in the centre of the clamping sha (Fig. 53a – top). Hence, the clamping sha must only transmit torque resulting from the cutting forces and the bending moment of the resultant cutting force which will be present. Typically, the outer insert’s re- sultant cutting force F A (i.e. Fig. 53b) is comprised of the following forces: • Remaining cutting force (∆F c ) – generated through greater wear rates at the periphery of the outer cut- ting insert, • Passive force (F p ) – generated by the corner radius of the outer cutting insert. With indexable drills, the chip ute is selected so that the drill’s prole from the tip up to the chip ute, has its runout twisted by between 65° to 85°. In the vicin- ity of the chip ute the runout (i.e. the longest ‘lever arm’ of the force), is where the maximum resistance 19 Some tooling manufacturers recommend that an indexable drill’s inner insert is positioned slightly below the spindle’s cen- treline, as this allows a small core of uncut material to pass over the top surface of the insert and break o – being carried away with the rest of the chips. 20 ‘Retraction striae’ refers to the ‘trail-lines’ resulting from the outer insert’s gouging, or ploughing the previously drilled sur- face as it is withdrawn from the hole. moment to the resultant cutting force F A is found. e bending strength attained in this manner can be greatly increased by employing round proled chip utes. is rounded chip ute cross-section, does not signicantly weaken the drill’s body and provides op- timum chip-ow – even when drilling long-chipping workpieces. A taper of the tool holder behind the in- sert seats, prevents a ‘squeezing’ of the chips between the drill and the drill-hole wall. Due to the design of the indexable drill, the two cutting inserts are subjected to very dissimilar stresses when drilling. For example, the indexable drill (Fig. 53a – bottom), has the outer insert being subjected to greater stress than its inner counterpart, typically having both thermal and abrasive stresses, while the inner insert must have high toughness characteris- tics. Some cutting tool manufacturers recommend so- called ‘mixed-tipping’ 21 of inserts, where a toughened grade is used for the inner insert and a wear-resistant grade for the outer insert. However, some discretion should be used when utilising indexable drills with mixed cutting inserts, so perhaps reference back to the tooling manufacturer may be advisable if produc- tion quantities are sucient in order to optimise this potential ‘mixed grade strategy’. Typically, by exploit- ing ‘mixed-tipping’ , for example when machining free-cutting steel grades cutting speeds of up to 400 m min –1 are possible, whereas when drilling low-silicon aluminium grades cutting speeds of 600 m min –1 can be achieved with tool lives of up to 45 min of cutting time per edge. Several design factors will inuence an indexable drill’s performance, these include: • Sintered cutting insert chip-breakers – these will improve chip control and enable high penetration rates to be utilised, • Advanced ute design – allowing deeper chip gul- lets, thus minimising chip-jamming tendencies, • Faster-, slower-, or straight-uted designs – with wider ute proles reduce chip-binding and degra- dation of the drilled hole surface, whilst also im- proving penetration rates, • Cutting insert shape – utilising square (i.e. having 4 cutting edges), rectangular (i.e. with two edges), or triangular inserts (i.e. having three edges) – the 21 ‘Mixed-tipping’ , refers to having dissimilar grades of inserts for the outer and inner cutting edges, as they full dierent mechanical working criteria whilst drilling. Drilling and Associated Technologies Figure 53. Indexable insert drills – insert position and ute geometry. [Courtesy of Kennametal Hertel]. latter being the most popular version for general- purpose drilling operations. NB Double-sided cutting inserts are available, but are mainly used for milling operations. One major advantage that indexable drills have over twist drills, is that they can be oset to produce dier- ent hole diameters. is oset for turning centres can be up to 3.8 mm, or 7.6 mm on a diameter 22 , which in reality, amounts to a ‘ne-boring’ operation, giv- 22 On machining centres this oset is somewhat less, with the max- imum radial oset being approximately 1 mm, or 2 mm on dia- meter. Of note, is that when an indexable drill is oset, then the maximum feedrate should be no greater than 0.15 mm rev –1 . Chapter ing considerable scope in diametric sizing of holes. ese drills should be selected for the minimum overhang and need to have both the drill’s and ma- chine tool’s centrelines parallel 23 to within 0.076 mm, or better. By utilising an indexable drill on a turning opera- tion, they can be benecial when attempting to start drilling angled, or uneven workpiece surfaces, as illus- trated by some of the surface faces depicted in Fig. 54. Indexable drills, employed for such non-at workpiece faces, obviates the need for either a previous counter boring, or spotfacing operation. e major advantage of indexable drills over either their HSS, or carbide twist drill counterparts, is their ability to run at much higher rates. For example and, by way of simplistic comparison, if a φ18 mm hole has to be drilled in free-cutting stainless steel, then an: • HSS twist drill – can be run at 16.8 m min –1 with a 0.2 mm rev –1 feed would produce 280 rpm at 57 mm min –1 , • Solid carbide twist drill – can be run at 61 m min –1 with a 0.1 mm rev –1 feed would produce 1,019 rpm at 103 mm min –1 , • Indexable drill – can be run at 170 m min –1 with a 0.76 mm rev –1 feed would produce 2,852 rpm at 217 mm min –1 . NB e indexable drill can be run up to 0.8 mm rev –1 and if one were to attempt to utilise a twist drill with the cutting data shown above, then it would either burn-out, or catastrophically fail in the endeavour. An important safety note when using these indexable drills in turning operations, is that the high penetra- tion rates, coupled to the exit of a through hole in the part, creates a slug which is thrown clear from either the chuck, or from the rear of the bore of the machine’s headstock at tremendous force. If the machine tool is not guarded appropriately, it could prove to be hazard- ous to any operator in the local vicinity. 23 Axis oset/misalignment: for example, if there is a 1° mis- alignment between these centrelines, then the centrelines will be 0.43 mm apart at 25 mm from the point at which the cen- trelines cross at the tool’s tip. .. Counter-Boring/Trepanning A counter-boring operation is an oen utilised tech- nique for the enlargement of previously manufac- tured holes, normally to provide them with accurate dimensions and/or improved surface nish (i.e. see Fig. 55a). Counter-bores are also produced to register a larger-faced shouldered sha, or to sink a precision cap-head bolt below the clamped part’s surface. In this latter case, oen the previously-drilled hole is used to align the bolt’s axis, by using a counter-boring tool with a ‘pilot-bush’ of the same drilled diameter to act as a guide to allow machining to a correct counter-bored depth. Counter-boring heads are employed to open-out existing holes (Fig. 55a). e heads were in the past, oen of a single insert design, but latterly, they are produced with multiple inserts – particularly when the working clearance is such, that it cannot cope with by the single insert variety. e multi-insert counter-bor- ing heads, can have their inserts nely diametrically- adjusted by means of precision wedge and radial ad- justing screws. A trepanning operation is undertaken in one op- eration, but instead of machining the complete hole, only part of the hole is cut, leaving a core (Fig. 55b). For large workpiece dimension machining, a trepan- ning operation uses less power and axial pressure than other equivalent manufacturing processes. However, one major problem occurs with the trepanning opera- tion, which is that the core produced as the trepanning tool penetrates into the workpiece, becomes quite dif- cult to handle (i.e. see Fig. 55b -lower diagram). Trepanning heads are normally utilised on: • Large workpiece diameters – greater than 120 mm, • Limited machine tool power – alternatively, if it is not prudent to switch to another machine tool and/ or lose part orientation and register 24 , • Core utilisation – large cores can be usefully utilised as precision material stock. 24 Part orientation and register, refers to the initial setup, where once the workpiece has been clamped and partially machined, it cannot be satisfactorily removed then reset without a loss of both accuracy and precision. Drilling and Associated Technologies Figure 54. A wide range of drilling/boring operations can be undertaken using indexable insert drills. [Courtesy of Seco Tools]. Chapter Figure 55. Counterboring and trepanning. [Courtesy of Sandvik Coromant]. Drilling and Associated Technologies .. Special-Purpose, or Customised Drilling and Multi-Spindle Drilling Most of today’s Special-purpose, or Customised Drills and Multi-spindle drills are normally designed and manufactured to meet the following criteria: • Long production runs 25 – these enable the extra cost of this purpose-designed and built tooling to amortised 26 over the cost of the production period, • Short cycle times 27 – when time is the ‘essence of importance’ in the production of the component, but it is not necessarily related to the overall quan- tity of the production run, • Tooling accuracy is reected in the part’s manufac- ture – if for example the precision part must have all its component features in accurate alignment, or in a specic relationship to a particular datum 28 : face, plane or point. Special-purpose, or Customised tooling is normally required if one, or several of the criteria mentioned above are to be met. To have a tooling manufacturer design special-purpose tooling to meet the production demands of manufacture, is not undertaken lightly, as for complex tooling, its: design, build and prove-out, prior to use, could prove to be expensive. However, many companies resort to this type of custom-built tooling, because it is the only way that the product can be manufactured economically. Oen, multiple fea- tures are incorporated into just one tool, typically for hole- and post-hole making operations, such as those depicted in Fig. 56a. A relatively simple example of this special-purpose tooling is illustrated in Fig. 56b, where three production operations for the manufac- ture of the female part of the pull-stud mechanism for a milling cutter toolholder is depicted, namely, drilling, 25 Long production runs usually refers to some form of continu- ous production , or large batch sizes, of > 5,000 components, to make the cost of the Special-purpose tooling viable. 26 Amortisation, refers to the ‘pay-back’ of the tooling over the ‘life’ of the production of the parts produced. 27 Short cycle times, are considered to be the quickest time that the part can be produced, under ‘standard’ machining conditions. 28 Datum – the term refers to origin of the measurements for the particular component feature, which could be from a face, plane, or point. chamfering and counter-boring. In this case, not only does the usage of special-purpose tooling here seem the obvious solution, as it combines these individual operations in one, it has the advantage of meeting all three of the production criteria listed above, with the added advantages of both using fewer tools and utilis- ing less space in the tool magazine. Some special-pur- pose tools are very complex in their design and quite sophisticated in operation, but their supplementary cost more than outweighs this by the production gains oered by their consequent implementation. Multi-spindle drilling 29 tooling is ideal when a series of hole patterns are required on a component, such as for specic congurations of: pitch circle diameters, hole grid patterns, line of holes, or a combination of these (i.e. see examples of specic patterns in Fig. 57 – top right). Hole pitch circle diameters can easily be accommodated, for large and small pitch diameters on the same tooling, Likewise, hole line and grid patterns can be quite diverse, within the diametral area of the ‘cluster plate’ (i.e. see Fig. 57 – top le). Multi-spindle drilling heads utilise a main drive gear which is engaged with an idler and then onto the drill spindle gear, this being attached to the individual drill (i.e. see Fig. 57 – exploded view of a typical sys- tem). e cluster plate orientates the individual drill spindles and their rotational speeds can be margin- ally increased, or decreased, by changing the driver- to-driven gear ratio, moreover, their rotational direc- tion can be changed by the introduction of another idler into the gear train. erefore, if additional idlers are present, to change the drill’s rotation, then the an appropriate le-hand drill would be required here. By purposefully modifying each drill’s rotational di- rection, this has the advantage of minimising overall torque eects on the multi-spindle drilling head, al- lowing a large number of drills to be utilised for one particular operation (i.e. see Fig. 57 – lower le-hand photograph). An important point in utilising multi- spindle drilling heads, is presetting their respective drill lengths, so that they engage with the workpiece’s surface at the correct height. By the correct production application of both Spe- cial-purpose and Multi-spindle drilling tooling, then 29 Multi-spindle tools, refer to more than one individual tool ro- tating in its respective toolholder, enabling several holes to be manufactured in just one operation. Chapter Figure 56. Special-purpose multi-functional tooling can be designed and manufactured to machine many part features simultaneously . Drilling and Associated Technologies Figure 57. The application of multi-spindle drilling heads to increase productive throughput. Chapter signicant economic savings can be made and their initial capital outlay will have been worthwhile. How- ever, such complex and expensive tooling used inap- propriately can be counter-productive, so consider- able thought and care should be made into any future implementation of these tools. .. Deep-Hole Drilling/ Gun-Drilling Deep-Hole Drilling – an Introduction Deep-hole drilling can be characterised by, high mate- rial removal rates, having excellent: hole straightness, dimensional tolerances and machined surface texture. Deep-hole drilling applications are utilised across di- verse industrial applications, including: aerospace, nuclear power, oil and gas, as well as for steel and chemical processing industries. ese industries place a high demand on all aspects of drilled hole quality and reliability, with components being very expensive, any failures will have severe economic consequences. e name Deep-hole drilling implies the machin- ing of holes with a relatively long hole depth to its diameter. Typically at the lower length-to-diameter ratios they can be as short as x5 the diameter, con- versely at the other end of the scale, ‘ratios’ of > x100 the diameter can be successfully generated, with close tolerances and a surface texture approaching 0.1 µm (Ra). ere are a considerable number of deep-drill- ing production techniques, with each one having an appropriate usage for a particular hole generation method. A typical deep-drilling tooling assembly is essentially ‘self-piloting’ , in that the cutting forces generated are balanced, not with respect to the cut- ting edges – as is the situation with Twist drills – but invariably, by pads that are situated at 90° and 180° to that of the cutting edge. ese pads rub against the bore’s surface being generated and therefore support the head, while burnishing 30 the surface. e machine tools enabling these deep-drilled holes to be generated can be expensive, along with the appropriate tooling, but the production costs can be dramatically reduced, by employing such a machining strategy. One of the 30 Burnishing will improve the surface nish and dimensional ac- curacy, by plastically deforming the machined surface layers – cusps – without removing any additional workpiece material. major problems of utilising Deep-hole drilling, is chip disposal, as the deeper the hole is drilled, the further the chip must travel from the cutting edge to the hole’s exit. is chip evacuation distance, can increase the probability of chip-jamming, or binding in the ute as the chip attempts to exit the deep-drilled hole. Notwithstanding the problems associated with BUE, which hinders the tool’s ability to break chips. Coolant control and its operational usage is important in any the Deep-hole drilling technique, as one of its main functions is – apart from lubricating/cooling the cut- ting edge and chip ushing – is to restrict frictional ef- fects between the: drill, chip and hole wall. Moreover, if friction builds-up due to poor coolant delivery, this can result in higher torsional eects, which may cause the drill to snap. Gun-Drills Gun-drills (i.e. see Fig. 58a), are normally utilised to machine small, straight diameters to high tolerances and having excellent nishes in a single operation. Drilled hole sizes can range from as small as φ1.5 mm to φ75 mm in a single pass, with depths equating to 100 times the tool’s diameter 31 . e ‘drilling system’ is a highly developed and ecient technique for produc- ing deep holes in wide variety of workpiece materials, ranging from: plastics, breglass, to high-strength ma- terials such as Inconel. is tooling usually consists of either a cemented carbide, or cemented carbide-tipped drill head tted to a tube-shaped shank 32 . e former solid carbide drill head version allows the tooling to be reground as necessary, while the latter version is normally employed for larger diameter hole drilling operations. e drill head has two distinct designs, either having a ‘kidney-shaped’ , or a cylindrical hole present, for the delivery of cutting uid, which pro- vides: • Flow of cutting uid – to create the maximum ow rate and chip-ushing, 31 ‘Special-purpose’ Gun-drills can be produced to generate drilled holes up to φ150 mm having 200:1 length-to-diameter ratios, at penetration rates of better than equivalent diameter Twist drills. 32 Gun-drills would as a rule, have their drill head’s brazed – via silver soldering – onto a tube-shaped shank, these in turn, are also brazed onto a ‘driver’ (i.e. of various designs) of the re- quired length for the successful drilling of long slender holes in the workpiece. Drilling and Associated Technologies . outer and inner cutting edges, as they full dierent mechanical working criteria whilst drilling. Drilling and Associated Technologies Figure 53. Indexable insert drills – insert position and. Seco Tools]. Chapter Figure 55. Counterboring and trepanning. [Courtesy of Sandvik Coromant]. Drilling and Associated Technologies .. Special-Purpose, or Customised Drilling and. clamped and partially machined, it cannot be satisfactorily removed then reset without a loss of both accuracy and precision. Drilling and Associated Technologies Figure 54. A wide range of drilling/ boring
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